Liquid crystal displays have been used in various display embodiments such as watches-and desk calculators because of their flatness, lightness, and low electric power consumption. With the recent advancement of integrated circuits (IC), liquid crystal displays have been increasing the display size and extending their use in computers, liquid crystal TV sets, etc. in place of conventional cathode-ray tubes. However, nematic liquid crystals which have conventionally been used have a slow response time of from 10 to 50 milliseconds and undergo a reduction in display contrast according as the number of pixels increases.
In the state-of-the-art liquid crystal displays, the above-described disadvantage is coped with by fitting each pixel with a thin film transistor (TFT) to achieve so-called active matrix driving or by increasing the angle of twist of liquid crystal molecules sandwiched between a pair of substrates to 220.degree. to 270.degree. (super-twisted nematic: STN).
Mounting of TFT according to the former means not only entails very high cost but has a poor yield, resulting in an increased production cost. Cost reduction by introducing a large-scale production line having been studied, there is a limit due to essential involvement of many production steps. Further, ever since the appearance of high-definition televisions (HDTV), there has been an increasing demand of liquid crystal displays making a high-density display. In nature of TFT and nematic liquid crystals, it is nevertheless considered very difficult to increase display density.
On the other hand, although the STN mode exhibits an increased contrast ratio, it has a slower response time of from 100 to 200 milliseconds and is thus limited in application.
It has therefore been keenly demanded to develop a liquid crystal element which achieves high-density displaying at a fast response time. Ferroelectric liquid crystal display elements form the nucleus of such expectations.
Ever since the report of N. A. Clark, et al. on surface-stabilized ferroelectric liquid crystal devices (SSFLCD) (refer to N. A. Clark, et al., Appl. Phys. Lett., Vol. 36, p. 899 (1980)), extensive studies have been directed to ferroelectric liquid crystals with the attention on their fast response. However, ferroelectric liquid crystal display elements have not yet been put to practical use due to problems of response time, molecular orientation, etc. still remaining unsolved. For example, the molecular orientation of ferroelectric liquid crystals proved more complicated than suggested by Clark, et al. That is, the director of liquid crystal molecules is apt to be twisted in smectic layers (spray state), under which a high contrast ratio cannot be obtained. Further, the layers have been believed to be aligned upright and perpendicular to the upper and lower substrates (bookshelf structure) but, in fact, were found to have a bent structure (chevron structure). As a result, zigzag defects appear to reduce a contrast ratio.
With respect to response time, it was believed in the early stage of studies that ferroelectric liquid crystal elements respond in several microseconds. In fact, however, the shortest of the so far reached response time is only several tens of microseconds. It is therefore required to further reduce the response time of ferroelectric liquid crystals for putting them to practical use.
Further, ferroelectric liquid crystals has another problem of low memory effects. That is, the liquid crystal orientation with an alternating electric field applied is readily destroyed after cutting the electric field. On account of this, liquid crystal displays utilizing scanning are accompanied with difficulties in obtaining a high contrast ratio or in greatly increasing the lines of scanning.
Memory effects are observed as a phenomenon resulting from a difference between the angle 2.theta. (.theta.: tilt angle with an electric field applied) formed between the first and second optically recognizable stable states appearing on application of an alternating electric field to ferroelectric liquid crystals and the angle 2.theta.' (.theta.': tilt angle during memory) formed between the third and fourth stable states appearing on cutting the electric field, the tilt angle during memory being considerably smaller than that with an electric field applied, as illustrated in FIG. 1.
While this phenomenon is deemed attributed to the above-mentioned spray state or chevron structure, etc., many points still remain obscure.
It is theoretically known that the intensity of the light transmitting through ferroelectric liquid crystals reaches the maximum at a tilt angle of 22.5.degree.. It has therefore been attempted to bring the tilt angle during memory close to 22.5.degree. from the aspect of both orientating materials and liquid crystal materials.
As an approach from the aspect of liquid crystal materials, it has been under study to improve memory effects by widening the tilt angle of ferroelectric liquid crystals with an electric field applied over 22.5.degree. thereby to widen the tilt angle during memory.
On the other hand, a tilt angle has great influences on response time in such a manner that too a wide tilt angle results in an increase of response time.
Considering that the shortest response time of ferroelectric liquid crystals reached to date is several tens of microseconds as stated above, widening of the tilt angle leading to an increased response time is of extreme disadvantage.
Accordingly, it has been keenly demanded to develop a ferroelectric liquid crystal composition having high memory effects, providing high contrast displays, and responding at a fast time while being free from the above-mentioned disadvantages associated with conventional ferroelectric liquid crystals and electrooptical elements using them.